Packets sent to downstream router gets blocked by the main one

[jagdtigger]

Hi all.

I kinda hit a brick wall with this one. I have a main router that is directly plugged into the cable modem, and then a second one behind this that will be used to create a strictly filtered access. Both LAN is on its own address range, static routes( no NAT on the second router) and firewall rules are in place to allow communication between the 2 LAN and a temporary allow all firewall rule until i build up the filter. Reaching the internet is no problem so the routing is working i assume. Here comes the interesting part. I can ping anything on the main LAN from behind the second router, but if a try to open for example the WebUI of any of my devices it times out. Looking at the log of the main router i can see that it blocks packets sent to the second router. Im totally lost on this...
The two router doesnt have a built in switch. Only the main one has one plugged into a managed switch, then a second managed switch is plugged into the 1st switch.

Hope someone will have an idea whats going on here and why it int work...

Thanks in advance :) .